In scanning systems with large numerical apertures the scanning lens sometimes has to be moved rapidly across an object in order to scan it, while maintaining precise focus. In those cases it is impractical to use the conventional "voice coil" type of focus servo actuator due to its size and the requirement of an electrical connection to the rapidly moving lens. This is true in particular when the scan distance is large relative to the size of the lens. The current invention overcomes this limitation by using a novel type of moving iron actuator which has a very low mass and requires no electrical connection to the moving part.